The present invention relates to a method of manufacturing a core to be used for a transformer or a reactor, for example, and, in particular, to a wound core formed by winding a long, narrow shaped strip of magnetic material.
Conventionally, for a transformer of low- or medium power of about 5-200 KVA, a wound core 1 having a stepped-circular shape in cross-section, as shown in FIGS. 6 and 7, is used. The wound core 1 is manufactured by winding three kinds of belt shaped material, each of different width. That is, a first material of narrow width is wound around a bobbin (not shown in the FIGS.) to form a portion 2a, and the winding operation is discontinued in order to connect a second material of medium width to the first wound material. Then the winding is resumed to form portion 2b, and the winding is discontinued again in order to connect a third material of broad width. Then the winding is resumed to form portion 2c. Thereafter, portion 2d is formed with the material of medium width and portion 2e with the material of narrow width in the same manner, respectively. Then the wound core is completed after removing the bobbin.
A dividable cylindrical bobbin is fitted around the leg portion of the wound core which has a stepped-circular shape in cross-section and, while the bobbin is rotating, wires are wound around the bobbin. The wound core is made into a transformer or other device.
In the wound core having a stepped-circular shape in cross-section, such steps are provided to increase the occupation ratio of the cross-section of the magnetic material in the wire wound circularly and at the same time enabling the core to cool itself by conducting a flow of air or oil through spaces formed by the steps. The occupation ratio of the cross-section is about 85% for a core with five steps as shown in the Figure. The more steps provided, the larger the occupation ratio that may be achieved. If a form having three steps, which is simpler, is provided, the occupation ratio is about 79%. In practical embodiments of a transformer, a wound core having three to nine steps may be used.
In addition to the wound core with steps as described above, a cut-core 3 with steps as shown in FIGS. 8 and 9 is available. In the cut-core with steps, the wound core is cut and separated after being formed, a coil 4 is put in, and the separated parts are joined to each other for use. The shape of the cross-section of the cut-core may be a square because the coil is put into the core. The corner portions of the square, however, are removed to form a cross-section shape with three steps due to a restriction to the bending radius of the coil wire.
In manufacturing either the aforementioned wound core having a stepped-circular shape in ross section or a cut-core with steps it is necessary to interrupt the winding operation and to connect a new strip of material each time it is required to change the width of the material. The efficiency of the operation is therefore reduced and completed products have problems such that the magnetic reluctance increases and noises are caused due to movements between the connecting portions. Further, it is complicted to control the materials because a number of different materials are used.
Alternatively a wound core with a circular cross-section is commonly used (Japanese patent publication No. 60-28357, Japanese patent publication No. 61-22851). To make such a wound core, a material is cut out so that the width changes gradually and the edge forms an elliptical curve, and then the cut material is wound.
Although a high occupation ratio of the cross-section can be obtained with the wound core with a circular cross-section, there is a waste of material and economical efficiency is therefore reduced because the side edge of the material supplied in the form of a belt shape of equal width is cut out into a curved form for use. Furthermore, the process of scrapping and cutting takes considerable effort. Thus, a wound core of this kind is normally used only for light electric devices of relatively small capacity and is not used for power instruments.
Furthermore, a wound core with a polygonal cross-section is known (Japanese patent publication No. 44-24605). This wound core is manufactured in the following manner. A belt shaped material is temporarily wound, and then shifted sideways to form the side end into a circular cone. It is then cut out in a plane and rewound to form a winding material in a shape of long narrow trapezoid. Several kinds of the winding materials are wound to a bobbin being connected to each other to form a wound core having a polygonal shaped cross-section.
The wound core with a polygonal cross-section has the advantage that the material waste is reduced, but because it has connected portions, there are potential problems in practical application as in the case of the aforementioned wound core with a stepped-circular cross-section. Further, the cutting of the material is troublesome as well as difficult. Therefore, the wound core of this kind has not been put to practical use.